Caustic soda and chlorine are produced concurrently with hydrogen by the electrolysis of an aqueous solution of sodium chloride in various types of electrolytic cells. Membrane cells using perfluorinated ion exchange membranes are the latest state of the art electrolytic cells utilized in the production of chlorine and caustic soda.
While membrane cells offer significant advantages over mercury cells and diaphragm cells, namely cell efficiency, power consumption, quality of products, and overall economics, these membrane cells require much purer brine than their predecessors. For example, calcium and magnesium content of the feed brine to the membrane cells must be reduced to the low ppb range.
Furthermore, the sulfate ion concentration has become very important in the feed brine to membrane cells, especially as the membranes are improved in composition to enhance their performance, because high concentrations of sulfate ions can cause premature failure of the membranes, both mechanically and in their separation performance. A typical feed brine sodium sulfate specification for membrane cells is below 7.0 gpl (4.73 gpl sulfate ion). Calcium sulfate impurity in many rock salts is greater than the 4.73 gpl sulfate ion specification. Further, only about 50% of the sodium chloride fed to the cells is converted, and the other 50% exits from the cells as depleted brine containing the sodium sulfate that was in the original cell brine feed.
The depleted brine cannot be thrown away because of economic and environmental considerations; hence, it must be resaturated, treated and returned to the cells. Since new calcium sulfate is dissolved in the resaturation process, the total sulfate ion concentration will keep increasing with each recycle step. The same is true if the cell plant is located near, or in pipe line communication with, the brine wells. Once the sodium sulfate content of the feed brine exceeds 7.0 gpl, the sulfate ions are considered to be detrimental to cell efficiency and membrane life. Thus, steps must be taken to eliminate or reduce the sulfate ions picked up in the process of dissolving salt or built up in the depleted brine sent to the resaturation process to make brine for electrolysis.